Monitoring methods are described, for example, in German Patent Application No. 36 21 937, Al, U.S. Pat. No. 5,063,383, and European Patent Application No. 0 579 235 A2. The methods described in these publications have in common that known analog test signals are applied in special test phases to the analog to digital converter (A/D converter) to be monitored, and that the digital values generated from these test signals by the A/D converter to be monitored are checked for correctness by comparing them to known nominal values.
During such check tests (test phases) of A/D converters, these cannot be used or, at most, only have a restricted use for "normal" A/D conversions. For this reason, in particular, the check test cannot take place continuously, but rather only in relatively large time intervals.
One way to rectify this problem would be to provide a more or less identical second A/D converter in parallel to the A/D converter to be monitored, and to check the digital values generated by the A/D converters for consistency. However, the practical realization of a monitoring method of this kind, especially because of the provision of a second A/D converter, requires considerable engineering outlay and is, therefore, expensive.